Morpho-anatomical and biochemical modifications in Lactuca sativa L. plants in response to increasing doses of High-LET ionizing radiation

The presence of unique environmental factors such as ionizing radiation (IR) in Space may be a major hindrance to the survival of terrestrial organisms. Therefore, understanding the impact of Space radiation on organisms is an essential part of exploration research to successfully inhabit Moon and Mars.

Given the pivotal role of plants in sustainable closed artificial ecosystems in Space (i.e. Bioregenerative Life Support Systems) as food producers and bio-regenerators of environmental resources such as oxygen and water, and, plant resistance to IR is undoubtedly one of the first trait to consider. Indeed, IR may influence plants growth and development, potentially affecting their bioregenerative performance, jeopardizing astronaut survival and in turn the success of future Space missions. Notwithstanding, available information on plants' responses to IR remain unclear, and intrinsic IR and plant factors (e.g., type of radiation, dose, plant species, cultivar, and developmental stage at the time of irradiation) interactions must be considered.

The purpose of this study was to test the radio resistance of the Salanova® lettuce (Lactuca sativa L. var. capitata) to different doses of high-LET (Linear Energy Transfer) radiation, in terms of morpho-anatomical and nutritional traits. At the GSI Helmholtzzentrum für Schwerionenforschung GmbH, dry seeds were exposed to increasing doses (0-control, 0.3, 1, 10, 20, and 25 Gy) of iron ions (56^Fe) (one of the most damaging ions in the galactic cosmic ray spectrum). Following irradiation, plants were grown in a growth chamber under controlled light, temperature, and relative humidity conditions.

Seed germination percentage, as well as plant growth and leaf photosynthesis performances were monitored during cultivation.

Morpho-biometric parameters such as plant total leaf area, number of leaves, and fresh and dry biomass were quantified at harvest. Leaf functional anatomical traits (e.g., lamina thickness, stomatal frequency and size, vein density) were also analyzed using light and epifluorescence microscopy and digital image analysis. Finally, to assess the phytochemical and nutritional profile, the antioxidant capacity, chlorophylls, carotenoid, and mineral composition content were determined.

Results showed that responses of Salanova lettuce from irradiated seeds changed depending on the IR dose delivered, with the occurrence of both hormetic and detrimental outcomes, as well as possible plant radioprotective strategies. This information will be helpful to unveil the mechanisms behind plant radiotolerance and to define the shielding requirements for Space cultivation facilities.

Part of the results presented here is based on the experiment Bio_08_DeMicco, which was performed at the SIS18 at the GSI Helmholtzzentrum fuer Schwerionenforschung, Darmstadt (Germany) in the frame of FAIR Phase-0.